Little is known about the developmental mechanisms that determine the size of the kidney and the number of nephrons. Why does the kidney grow to a particular size and complexity, and then stop? Is there an intrinsic limitation in the number of divisions of progenitor cells, or is organ size regulated by some other mechanism, such as external signals? In this Revision application, we propose new experiments to address these questions. Two major renal cell lineages are the collecting duct epithelia, which derive from the ureteric bud, and the nephron epithelia, which derive from the metanephric mesenchyme. Cells expressing the Ret receptor, located at the tips of the ureteric bud, are the major progenitors of collecting ducts, while metanephric mesenchyme cells expressing the Ret- ligand GDNF are the progenitors of nephron epithelia. We will use mice expressing an inducible form of Cre recombinase (Cre-ERT2) under the control of the Ret or Gdnf genes. These mice will be crossed with a strain that conditionally expresses a cytotoxic gene, DTA, only in cells that express active Cre, and in their descendants. By inducing Cre activity with Tamoxifen, we will selectively reduce the number of the progenitor cells in the ureteric bud or metanephric mesenchyme, at specific stages of kidney development. We will then analyze the kidneys in vivo and in organ culture, to ask if this alters the rates of ureteric bud growth and nephrogenesis, and the final kidney size and nephron number. If the progen- itors are programmed to divide a limited number of times, then destroying a fraction of them should result in decreased organ size. Alternatively, if these cells have excess proliferative capacity and are regulated by external signals, the organ may recover and reach its normal size. Either the number of ureteric bud or nephron progenitor cells, or both, might limit the growth rate and final size of the kidney. Our experiments also address the basic question of whether tip cell number might determine the rates of elongation and branching of the ureteric bud, and similarly, how the number of nephron progenitors might affect the rate of formation and the size of nephrons. Understanding how the kidney achieves its normal size and nephron number has very important clinical implications, as reduced nephron number may favor the progression of renal diseases and hypertension. The proposed studies expand the scope of our original project (whose Aims concern the regulation of kidney development by growth factors, tyrosine kinase receptors, and intracellular signaling mechanisms) to include studies on the role of progenitor cells in kidney growth and development. It will allow for job creation by providing the funds to hire new postdoctoral researchers, and to purchase additional supplies and services. PUBLIC HEALTH RELEVANCE: Understanding how the kidney achieves its normal size has important clinical implications, as defects in organ growth during fetal development can lead to a reduction in the number of nephrons, the filtering units. This, in turn, may promote the progression of renal diseases and hypertension. This proposal investigates the mechanisms that control how the kidney grows to the correct size, and how the proper number of nephrons is formed.